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Thread: sanding aluminum

  1. #1
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    Post sanding aluminum

    I quite often use aluminum in projects where the look of it is important. Today, I edged a board for a sliding shelf with aluminum channel. To make it look better, I sanded the pieces. Got rid of the scratches, but there are these annoying 'spots' showing up on the freshly sanded surface. Thinking I was getting fingerprints on it, I carefully cleaned, then went at it again. I can't seem to be able to sand the 'marks' away, they just sort of pop into existence during sanding. They look like fingerprints, about that size. I've tried water and oil with the sandpaper, of which I have tried at least 3 different types. I can't get a consistent surface. I've done this before, sanding al, and sometimes, but not always, this problem comes up. Does anyone know what is happening here?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  2. #2
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    Could be a couple of things.

    Freshly-sanded aluminum is pretty sensitive to oil and fingerprints, they show up like sore thumbs. Water spots, perhaps.

    They could be minute dents in the surface, amd the sanding shows them up. If you're using a wood-type sandpaper and/or an orbital/air sander, the spots could be paper backing smears, spots where the pad spun too long, or similar marks.

    If you're hand-sanding with just your fingers, you're almost invaribly sanding "grooves" or spots of varying pressure.

    Get yourself some 320-grit wet-and-dry automotive stuff, and a 3M rubber 1/4-sheet sanding block. Fill a 5-gallon bucket about halfway with warm water and add a few drops of dish soap (I prefer a dab of Go-Jo, it seems to help cut through oil traces better.)

    Keep both block and part dripping wet and rinse often. If you're looking to make a smooth finish to buff and polish later, alternate directions. Sand one direction, then cross over that line by 60 to 90 degrees for a few strokes.

    If you want a "brushed" finish, always sand in a straight line.

    Rinse it off afterward with soap and warm water, and dry it with a clean towel.

    Finish as necessary, as the sanded surface will take and hold fingerprints, oil spatter and what have you way too well.

    Doc.
    Doc's Machine. (Probably not what you expect.)

  3. #3
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    .

    [This message has been edited by pgmrdan (edited 03-08-2004).]

  4. #4
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    If that channel was intended for edging it was probably anodized. You may well be sanding through the anodizing in some places and not others.
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  5. #5
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    I wish it was anodized, I wouldn't be having this problem. Customer tried to erase some of the spots after the piece was mounted, and handle installed. It got worse. Will be looking at it later.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  6. #6

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    Doc Nickel:
    Best sanding block I have found is from www.leevalley.com - it is a rock maple block with steel backed felt pad - also 1/4 sheet. Steel spring tighen the paper as the knob on the back of the block is screwed down. I would not dunk it in water though.

    I find green 3M pads in a random orbit sander give a nice frosted finish to Aluminum and Stainless both.

  7. #7
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    It is not common but sometimes the grain boundrys in the aluminum show up. We had this happen in the rolling mill several times when rolling foil. The sheet was covered with grain boundries that were very large, 8" or so in size, interesting pattern but scrap. You can't make it happen on purpose but it happens. I don't know that you can make it go away but you can polish it and hide the grain. The grain boundries are larger than they should be and therefore are visible. You are dealing with and extrusion and if the temperature was not right it will change the charactoristics of the aluminum. Sometimes a mild acid will also bring out the grain boundries so that they are visible on a polished surface but they are normally smaller than what you describe. If it is grain showing the color will be a little different on each of the marks.

    Hope this helps.

    Joe

  8. #8
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    I tried again today on a piece off the same length of tubing to sand it to a decent brushed finish. I couldn't get the marks to show up. They are consistently in the same place on the pieces that went on the cabinet. Maybe something on the sanding block deposited on the metal, and stained it. Maybe something in the metal, an inconsistency in the alloy? I've tried freehand, sanding block, flexible sanding block, some different papers and grits. I haven't tried emory paper yet. The best result I could get was using a fairly fine grit of paper, and some oil. Clogged the paper but left the piece looking not too bad. I even tried to imprint fingerprints on to the freshly sanded surface. I couldn't get the spots to appear that way. Job is done anyway, thanks all for the help.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  9. #9
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    Scotch brite and simple green. Rinse with water and blow dry. Where gloves so you dont get finger prints.

  10. #10
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    Jeep, I'm not familiar with simple green. What is that?
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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